Market Efficiency in an Irrational World

NBER Working Paper No. 7489Issued in January 2000NBER Program(s):Asset Pricing

This paper explains why investors are likely to be overconfident and how this behavioral bias affects investment decisions. Our analysis suggests that investor overconfidence can potentially generate stock return momentum and that this momentum effect is likely to be the strongest in those stocks whose valuation requires the interpretation of ambiguous information. Consistent with this, we find that momentum effects are stronger for growth stocks than value stocks. A portfolio strategy based on this hypothesis generates strong abnormal returns that do not appear to be attributable to risk. Although these results violate the traditional efficient markets hypothesis, they do not necessarily imply that rational but uniformed investors, without the benefit of hindsight, could have actually achieved the returns. We argue that to examine whether unexploited profit opportunities exist, one must test for what we call adaptive-efficiency, which is a somewhat weaker form of market efficiency that allows for the appearance of profit opportunities in historical data, but requires these profit opportunities to dissipate when they become apparent. Our tests reject this notion of adaptive-efficiency.